In general there are three types of GMA welding processes differentiated by their arc characteristics and the manner in which metal is transferred from the consumable electrode to the work piece.
The first of such metal transfer processes is known as Spray Arc wherein metal is transferred from the end of the electrode to the work piece or molten puddle in a stream or series of small molten droplets. The Spray Arc transfer occurs at what have heretofore been considered to be relatively high current densities but not generally exceeding 130,000 amperes/square inch (electrode cross section area) and with a weld deposition rate of 0.12 to 0.30 pounds/minute. Typically, electrode wire diameters vary from 0.030 inch to 1/16 (or 0.062) inches with voltages of from 15 to 36 volts. The transfer is normally in an argon or argon-rich gas.
The next process is referred to as Globular transfer wherein a relatively large globule forms at the end of the electrode and falls to the work piece when the force of gravity overcomes the surface tension of the molten drop. As the globule is transferred across the arc, it is subjected to forces in the arc and takes on an irregular shape and rotating motion. This sometimes causes the globule to reconnect with the electrode and the substrate and causes a short circuit which momentarily extinguishes the arc. Globular transfer occurs at lower current densities than with Spray Arc and can take place with a variety of shielding gases.
The third process is known as Short Circuit transfer which, because of relatively low transfer heat, is particularly useful in welding thin sections. In the Short Circuit transfer a globule of liquid metal forms at the end of the electrode and gradually elongates and moves toward the work piece until it makes contact to create a short circuit. The metal transfer is again by gravity and surface tension. When the globular bridge between the electrode and work piece is broken by a pinch force, the arc is interrupted after which the arc is renewed and the cycle begins again. Normally this type of transfer occurs in carbon dioxide, argon/carbon dioxide or helium base shielding gases. If employed with high current densities and normal arc voltages, the metal transfer by this process is much more violent causing the filler material to splatter in such a way as to create a weld that is unsatisfactory in appearance and geometry.
The metal transfer characteristics of the three processes may be summarized as follows:
______________________________________ Process Material Transfer ______________________________________ 1. Spray Arc extra small droplets - (long arc) no short circuiting 2. Globule large drops - random (long arc) short circuiting possible 3. Short Circuit small droplets - short (short arc) circuiting ______________________________________